Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data that are readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks that extend circumferentially around the disk. Each data track can store data in the form of magnetic transitions on the disk surface. For example, each transition can represent a bit of information.
A head includes an interactive element, such as a magnetic transducer, that is used to sense the magnetic transitions to read data, or to conduct an electric current that causes a magnetic transition on the disk surface, to write data. The magnetic transducer includes a read/write gap(s) that positions the active elements of the transducer at a position suitable for interaction with the magnetic transitions on the surface of the disk, as the disk rotates.
It is expected that users of disk drives will place ever greater demands on disk drive manufacturers with regard to the amount of data that can be stored in and rapidly retrieved from disk drive products. Modern software programs include graphics and other data structures that dramatically increase the amount of data that need to be stored. In addition, the rapid growth in the use of servers on computer networks requires large storage capabilities to accommodate the data needs of a large number of users on the network who utilize the servers.
Accordingly, recent disk drive research and development efforts have focused on the need to continually improve, among other things, the magnetic media used in the disks so as to substantially increase the storage capacity of each new disk drive design to levels sufficient to accommodate the ever greater demands for storage capacity placed on disk drive products by users. Moreover, the trend in recent years has been to design and build disk drive products that are lightweight and compact in size, and that operate at high rotational velocities of the disks to increase data read and write rates.
A consequence of the increasing capacity of disk drive products having compact designs is that the data density on the surface of the disks and the rotational speeds of the disks are approaching levels that are too high relative to the capability of conventional magnetic transducers to rapidly and accurately sense closely spaced, fast moving magnetic transitions in a data read operation. Moreover, conventional electronic circuits typically used to receive and process electrical signals representative of the transitions sensed by the magnetic transducers are also unable to operate at data read rates commensurate with the high data densities and rotational speeds of modern disk drive designs.
One proposal to meet the data retrieval requirements of modern disk drive designs is to utilize a magnetoresistive transducer (MR transducer) coupled to an electronic read channel that implements signal processing techniques such as partial response, maximum likelihood detection (PRML read channel). These components provide significantly improved performance capabilities and are able to process signals representative of data at rates suitable for operation with modern high capacity, high performance disk drives.
In an MR transducer, a magnetoresistive element is used to sense the magnetic transitions representing data. The magnetoresistive element comprises a material that exhibits a change in electrical resistance as a function of a change in magnetic flux of a magnetic field applied to the element. In a disk drive environment, the MR element is positioned within a transducer gap, above a disk surface. In this position, the electrical resistance of the element changes in time as magnetic transitions recorded on the disk pass beneath the gap, due to rotation of the disk. The changes in the resistance of the MR element caused by magnetic transitions on a disk occur far more quickly than the response of conventional transducers to magnetic transitions. Thus, an MR transducer is able to sense magnetic transitions at higher rotational speeds and data densities.
The MR transducer is coupled to an electronic circuit, a pre-amplifier, that operates to detect the resistance changes of the MR element, and generate electrical signals that vary in time as a function of the resistance changes. The pre-amplifier output, therefore, comprises an electrical signal that corresponds to the data recorded as magnetic transitions on the disk surface. The output of the preamplifier is coupled to a read channel that thereafter processes the pre-amplifier output signal according to PRML techniques to interpret the data represented by the output signal.
The PRML channel may comprise a read channel implementing any number of partial response techniques including, e.g., PR4ML, EPR4ML and EEPR4ML. PRML techniques can operate with more efficient data recording codes, and are able to process signals at more rapid rates than conventional peak detectors now widely used in disk drives to detect data from signals received from a transducer.
In any signal processing electronic circuit, such as a PRML read channel, the electrical signal output by the circuit varies as a function of the electrical signal input to the circuit. For example, in a simple amplifier, the signal output by the amplifier should have the same wave shape and frequency as the signal input to the amplifier, but with a higher amplitude. In other words, the amplifier amplifies the magnitude of the input signal.
The change between the input and output signals is referred to as the transfer function of the circuit. In the case of an ideal, simple amplifier, the transfer function can be expressed as output=k*(input), where k is a coefficient representing the magnitude of amplification. If the amplifier amplifies an input signal by a factor of 10, then k, in that instance, equals 10, and a one volt input signal will result in a 10 volt output-signal.
Actual electronic circuits do not, however, operate in the ideal fashion that they were designed. An amplifier will introduce some "noise" into the output signal such that the output does not necessarily have the exact same wave shape and/or frequency phase as the input signal. Generally, "noise" is any unwanted electrical changes introduced into a circuit's output that may result form, e.g., certain operating characteristics inherent in the circuit design, the operating environment of the circuit, and so on.
Electronic circuits that implement or support PRML signal processing techniques, particularly digital PRML channels, are complex and include numerous operating parameters and characteristics that affect circuit output, and thus the accuracy of data interpretation. Integrated circuits that implement PRML channels include variable parameters that can be adjusted to calibrate the PRML circuit for optimized operation. In other words, the transfer function of the PRML channel can be modified to minimize errors in data processing. The accuracy of calibration of the PRML channel is of utmost importance in achieving optimized performance for the disk drive.
Typically, each particular PRML channel is calibrated for optimized operation when coupled to the specific interactive element/disk interface of the disk drive where the channel is installed. Each interactive element/disk interface is unique, and the calibration of an associated PRML channel should correspond to the actual operating environment where it is to operate. Thus, in a multiple disk drive, the number of calibrations that must be performed can be considerable resulting in time consuming calibration procedures.
A conventional approach to calibration of a read channel is to write a test signal on the disk, using, e.g., a pseudorandom signal generation algorithm. A bit pattern corresponding to the random signal generated at any one time will be known upon generation, and the disk drive is operated to write the known generated signal onto the disk of the drive. The random test signal serves as a proxy for data that would ordinarily be written as magnetic transitions on the disk. The disk drive is then operated to cause the interactive element, e.g., the MR element, to read back the written test signal, using the PRML channel to interpret the magnetic transitions representing the test signal. The PRML channel outputs a bit pattern representing the written signal.
A bit by bit comparison can then be made between the known bit pattern of the test signal and the bit pattern output by the PRML channel, to determine an error rate, that is the number of bits in the PRML output that do not match the corresponding bit values of the known test signal. Of course, an ideal PRML channel would output a bit pattern that exactly matches the bit pattern of the test signal. Calibration of the PRML channel involves adjusting and modifying the operating parameters of the PRML channel to obtain a minimum error rate. A problem with the bit by bit comparison to measure an error rate is that a well designed PRML channel has a very low signal to noise ratio, and, therefore, a very low error rate, even within the range of parameter values that would be modified by a calibration of the channel.
For example, a PRML channel can have an error rate in the range of 10.sup.-9. A 10.sup.-9 error rate indicates that one billion bits must ordinarily be read from a test pattern before an error is encountered in one bit. Thus, the calibration process would involve a read back of bits numbered in the billions to accurately measure an error rate. This high bit read back amount results in very time consuming calibration procedures since a large number of bits must be read and decoded by the PRML channel in order to acquire enough bit error statistics to support an effective and accurate calibration of the channel.
Another approach is to install adaptive circuits in the read channel to provide measured values relating to channel performance. The values can be used to adjust or adapt PRML circuit parameters. A problem with this solution is that the values measured by the adaptive circuits are only loosely correlated to a channel error rate, and thus provide less than a fine tuned calibration for the channel. In addition, such adaptive circuits are generally too costly in terms of circuit complexity.